The invention relates to a device for transmitting torque between two parts rotatably mounted relative to one another. The device comprises a friction coupling with friction elements rotationally fixed to each of the rotatable parts. The friction coupling is loadable by at least one movable piston which, on one side, delimits a pressure chamber filled with a viscous fluid. The pressure chamber is connected to a reservoir. The pressure chamber is formed by a rotational housing and the piston. The rotational housing is connected to one of the rotational parts. A rotational member rotates in the pressure chamber and is connected to the other rotatable part. Rotational faces of the rotational member, and counter faces of a control member which is positioned in the pressure chamber constitute at least one shear channel. The shear channel is formed by a shear groove which is laterally delimited by walls, and by a surface covering the shear groove and being rotatable relative thereto. The shear groove extends between two ends in the circumferential direction. The control member is rotatable to a limited extent between two end positions relative to the rotational housing. The shear groove is connected to the reservoir and to the pressure chamber by control apertures in the control member, positioned at the end of said shear groove. The connection is effected such that, in the two end positions of the control member, a control aperture positioned at the front end of the shear groove (when viewed in the direction of relative rotation), communicates with the reservoir and a control aperture positioned at the rear end of the shear groove (when viewed in the direction of relative rotation) communicates with the pressure chamber.
Devices of the type herein described are used in the drivelines of motor vehicles and land machinery for generating a locking moment between two parts rotating relative to one another. The locking moment is dependent on the speed differential.
One application concerns differential drives wherein the coupling is arranged between parts which rotate relative to one another when differential processes occur in the drive. The coupling provides differential drives with a partial locking effect. The differential drives are preferably axle differentials, or central differentials, in motor vehicles.
Another application concerns motor vehicles with at least two driveline branches for a plurality of driven axles. The coupling is arranged directly in the driveline branch associated with one of the axles. As a result, this driveline branch is subjected to torque of a differential speed of the axle of said driveline branch and another directly driven axle by means of the coupling, whereas at identical speeds of the axles the coupling causes the respective driveline branch to be torque-free. The axle of said driveline branch is then dragged along by the vehicle.
Couplings of the above-mentioned type which are capable of sensing speed differentials are clearly advantageous as compared to torque sensing systems, with respect to both traction and driving dynamics.
A device of this type is described in DE 43 43 307 A1. The coupling operates by a pressure drag flow principle based on the shear effect of a viscous medium between two plates moving relative to one another. With relative movement, part of the medium--with reference to one of the plates--is conveyed in the direction of movement of the other plate. A gap between the two plates extends substantially parallel to the direction of relative movement and constitutes a shear groove when the gap is closed on its sides and delimited at two ends and covered by a moveable surface in the direction of the groove, said gap is forming a shear channel. As a function of the magnitude and direction of the relative movement, the shear channel conveys fluid from one end of the shear channel to the other end. The conveying pressure is directly proportional to the length of the shear channel, the viscosity of the sheared medium and the shear rate, i.e. the relative speed. If the shear channel is arranged to connect two chambers, and the two parts which form the shear channel are connected to each of the rotating parts of a coupling, a conveying pressure is generated which is dependent on the differential speed. The conveying pressure can increase the pressure in the pressure chamber. The pressure acts on at least one piston which loads the friction elements of a friction coupling. In this case, the at least one shear channel is recontrolled to generate pressure in both directions of relative rotation independent of the direction of rotation of the parts relative to one another. It is possible, at the time of recontrolling, to connect the shear channel end previously used for providing pressure directly to the reservoir, so that the pressure release at that end does not take place along the entire length of the shear channel. Further, different characteristics of the assembly dependent on the direction of rotation can be achieved in that in one of the two directions of relative rotation of the parts only part of the length of the shear channel for building up pressure is used. This is accomplished by providing an additional connection from the reservoir to the shear channel from an aperture positioned between the shear channel ends. The aperture is open in only one direction of relative rotation and is closed in the other direction of relative rotation.
Furthermore, to achieve different characteristics of the assembly dependent on the direction of rotating, a direct connection can be provided between an aperture and the pressure chamber to shorten the effective channel length for pressure build-up in which the aperture is closed in the opposed direction of relative rotation and positioned between the shear channel ends.
Finally, it has already been proposed to correct the effective forces. The correction depends on the desired curve of the locking moment generated by the friction coupling as a function of the speed differential. For this purpose, spring means may be supported on the housing to load the friction elements on the side facing the piston. Spring means may also be supported on the housing to load the piston on the side facing the friction elements.
The latter embodiments substantially achieve only linear characteristics with respect to the piston force as a function of the speed differential. For certain applications such as using the coupling as an axle differential in a front wheel drive vehicle or as a transfer coupling in a four wheel drive vehicle other characteristics may be desired which are not achievable.
It is therefore the object of the subject invention to provide a coupling having different characteristics of the assembly for use in a variety of applications.